There was another recent post , where the electric " Ranco " water valve had gone south. The valve was near the heater box at the front of the rig, the valve shuts off water flow through the heater core when MAX A/C is used so the A/C and the heater don't fight.Your valve may be Vacuum controlled , but you should have one.
If the hoses feel empty,when the engine is running at temp, you may have something as simple as a rad cap not holding pressure, or a low coolant level , but most coaches have a sensor for that.. Use caution removing the cap, wait till cool, and take it to an auto parts store for a test.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.